The objective of this proposal is to use synchrotron data to determine the structure of the regulatory (R) subunit of cAMP-dependent protein kinase (cAPK). Our goals are to understand the molecular basis of cAMP binding and cAMP mediated dissociation of the holoenzyme. At SSRL we were able to obtain higher resolution diffraction data on the native and three heavy atom derivatives. The electron density map computed revealed 90% of the structure. We need to collect additional data to complete the structure.